Memorandum

TO:

All Design Section Staff

FROM:

Chuck Ruth

DATE:

August 8, 1997

SUBJECT:

Peak Ground Acceleration Map Used for Bridge Design

The attached Peak Ground Acceleration Map replaces the current Velocity-Related
Acceleration Map (Figure 4.1.5-1) and supersedes my memo of August 10, 1995 (copy
attached).
The BDM will be updated at the next appropriate revision date as follows:

4.1.5 Earthquake Loads

d. Use the USGS Peak Ground Acceleration map (Figure 4.1.5-1, 10% Probability of
Exceedance in 50 Years) to obtain an acceleration coefficient for preliminary
design. The project Foundation Report will contain the
acceleration coefficient for preliminary design to use in the final design of a bridge..
When using Figure
4.1.5-1, interpolate between contours to find the value to use for a particular site, and
round to the nearest 1% of gravity(g). In general, Figure 4.1.5-1 can also be used
for bridge seismic retrofit designs. However, seismic evaluation and retrofitting of
older bridges can sometimes result in excessive costs (there retrofit costs are not
consistent
with the benefit gained). In these situations, the Bridge Design Engineer should be
consulted for direction.